INTRODUCTION: The day of the week or time of day that surgery is performed may influence postoperative mortality or complications. We aimed to examine whether surgery under general anaesthesia performed after-hours was associated with increased rates of mortality and morbidity, compared with surgery performed in-hours. METHODS: This population-based cohort study obtained data for patients who underwent surgery under general anaesthesia from the National Health Insurance Service of South Korea. Propensity score-matched groups of patients who underwent surgery either in-hours (weekdays between 9.00 and 18.00) or after-hours (weekdays between 18.00 and 09.00 or on a weekend/holiday) were compared for 90-day and 1-year mortality, and the incidence postoperative complications. RESULTS: A total of 1,416,844 patients were considered (63,567 in the after-hours group and 1,353,277 in the in-hours group) and after a 1:5 propensity score matching, 281,717 were included (57,497 in the after-hours group and 224,220 in the in-hours group). Patients in the after-hours group showed 3.58-fold (OR 3.58, 95%CI 3.47-3.69, p <
0.001) and 2.51-fold (hazard ratio 2.51, 95%CI 2.46-2.57, p <
0.001) higher 90-day and 1-year all-cause mortality rates, respectively, compared with those in the in-hours group. Patients in the after-hours group had a 2.14-fold (OR 2.14, 95%CI 2.10-2.19, p <
0.001) greater incidence of postoperative complications compared with those in the in-hours group. DISCUSSION: After-hours surgery was associated with a higher risk of death within 90 days, an increase in all-cause mortality after 1-year and a higher incidence of postoperative complications.